In recent years, there has been proposed a high-density optical information recording and reproducing apparatus based on the principle of holography as disclosed in JP-A No. 2008-123627. In such the optical information recording and reproducing apparatus, in the information recording mode, a light emitted from one light source is split into an information beam containing information to be modulated by a spatial light modulator (SLM), and a reference beam. The two beams having been split are incident on a recording medium from different directions, and the information can be recorded in the form of an interference fringe. On the other hand, in the information reproduction mode, the same reference beam as that in the information recording mode is radiated onto the recording medium having information recorded thereon, and the interference fringe is read, whereby the information having been recorded is reproduced.
The optical information recording and reproducing apparatus based on the principle of holography described above records information using an interference fringe made up of an information beam and a reference beam. Thus, the relationship between the information beam and reference beam at the time of recording and reproduction is crucial. To put it more specifically, for example, if the reference beam has a state different from that used at the time of recording information and it is incident on the recording medium for reproducing information, the information may not be recorded correctly. The different state of the reference beam in the sense in which it is used here includes a difference in a wavelength of a light source, a difference in an angle of a beam incident on a recording medium, and variations of amplitude or phase distribution in the reference beam.
Incidentally, it can be said that the wavelength of a light source and the amplitude and phase distribution in the reference beam depend on the characteristics of each individual semiconductor laser and are stable to a certain extent. By contrast, the angle of the beam incident on a recording medium tends to be changed from a default setting value by being affected by a vibration and environmental temperature variation at the time of transporting or using an optical information recording and reproducing apparatus, and chronological changes, for example. In the meantime, when the output intensity of the semiconductor laser is to be employed maximally for the intensity of information beam, a step is taken in some cases to ensure the light-flux diameter of the information beam will be approximately the same as that size of the SLM, or will be slightly greater than that size of the SLM for the purpose of a margin of safety. In this case, however, if the optical axis of the optical path of the information beam has made a parallel displacement in excess of the margin of safety with respect to the position at the time of manufacturing for some reasons, a part of the information beam will not enter the SLM, with the result that the beam is not effectively used.